Several microfabrication processes call for the use of very thick layers of photoresist. Typically, photoresist layers are applied to wafers by a spinning process. An amount of photoresist is applied in liquid form to a wafer, and the wafer is spun at a predetermined speed for some period of time to spread the photoresist across the wafer surface. A common problem for thick layers is the formation of an edge bead, which is a marked increase in thickness near the outer edge of the wafer. Significant variation in the photoresist thickness creates problems in exposure and development of photoresist during lithography as well as variations in finished die characteristics across a given wafer. In particular, when using a contact mask, a large edge bead can prevent direct contact across a significant area of the wafer, reducing resolution in lithography.
FIG. 1 shows a graphical representation of the edge thickness for three inch and four inch wafers on which a very thick layer has been spun in a prior art process. The dashed lines indicate illustrative upper and lower acceptable bounds for the photoresist layers. The acceptable bounds may vary widely from one process to another. The useful area 10 for the four inch wafer has a diameter of about 2.375 inches, while the useful area 12 for the three inch wafer has a diameter of about 1.5 inches. Approximating the useful areas 10, 20 as circular, the percentage of the four inch wafer area that is useful area 10 is about 35%, while the percentage of the three inch wafer area that is useful area 20 is about 25%. A further problem is that a limiting factor on edge bead size is the centrifugal force on the photoresist during spinning; for small wafers, the centrifugal forces generated are reduced by reduced radius of the wafer edge versus a larger wafer. These low percentages reduce chip yield from each individual wafer, increasing production costs by any of several measures, including wasted goods, environmental harm, and extra time in terms of machine usage and personnel hours.
In several processes, including, for example, a number of vertical cavity surface emitting laser (VCSEL) fabrication processes, thinned wafers are used. For example, rather than using a typical 500 micron thick wafer, some processes use wafers that are about 300 microns thick. These thinned wafers tend to be fragile. Further, some specialized processes, including VCSEL fabrication, make use of special wafers that are relatively small in comparison to the large wafers that many new microfabrication process machines are made to process. While many Si wafer processes for integrated circuitry now use or are configured to use eight inch or larger wafers, specialty processes such as some research and development as well as VSCEL fabrication processes make use of three, four or six inch wafers.